The invention relates to polyisocyanates containing allophanate groups and silane groups, to a process for preparing them and to their use as a starting component in the production of polyurethane polymers, more particularly as a crosslinker component in polyurethane paints and coatings.
Polyisocyanate mixtures containing alkoxysilane groups have been known for some time. Products of this kind, which in addition to the isocyanate group contain a second structure which is reactive, i.e. is capable of crosslinking, have been used in the past in different polyurethane systems and polyurethane applications for the purpose of obtaining specific properties—for example, for enhancing the adhesion, chemical resistance or scratch resistance of coatings.
For example, WO 03/054049 describes isocyanate-functional silanes, prepared from low-monomer-content aliphatic or cycloaliphatic polyisocyanates and secondary aminopropyltrimethoxysilanes, as adhesion promoters for polyurethane hotmelt adhesives.
In accordance with the teaching of JP-A 2005015644, as well, it is possible to enhance the adhesion of adhesives and sealants by using isocyanate prepolymers or polyisocyanates modified with N-substituted, i.e. secondary, aminopropylalkoxysilanes.
EP-B 0 994 139 claims reaction products of aliphatic and/or cycloaliphatic polyisocyanates with deficit amounts of alkoxysilane-functional aspartic esters, of the kind described in EP 0 596 360 as co-reactants for isocyanate-functional compounds, and, if desired, polyethylene oxide polyether alcohols, as binders for one-component moisture-crosslinking coatings, adhesives or sealants featuring accelerated curing.
Reaction products of aliphatic and/or cycloaliphatic polyisocyanates with deficit amounts of alkoxysilane-functional aspartic esters or secondary aminoalkylsilanes are also described in WO 02/058569 as crosslinker components for two-component polyurethane adhesion primers.
EP-B 0 872 499 describes aqueous two-component polyurethane coating materials comprising as their crosslinker component compounds containing isocyanate groups and alkoxysilyl groups. The use of these specific polyisocyanates leads to coatings combining enhanced water resistance with high gloss.
Hydrophilically modified polyisocyanates containing alkoxysilane groups, which are therefore easier to emulsify, have likewise already been identified as crosslinker components for aqueous 2K (2-component) coating and adhesive dispersions (e.g. EP-A 0 949 284).
Recent times have seen the proposal, as a crosslinker component for improving the scratch resistance of solvent-borne, thermosetting, 2K PU automotive clearcoat and topcoat materials, of reaction products of aliphatic and/or cycloaliphatic polyisocyanates with N,N-bis(trialkoxysilylpropyl)amines (PP 1 273 640).
A feature common to all of these polyisocyanate mixtures containing silane groups is that they are prepared by proportional reaction of unmodified polyisocyanates or polyisocyanate prepolymers with organofunctional silanes containing groups that are reactive towards isocyanate groups, examples being mercapto-functional silanes, primary aminoalkylsilanes, secondary N-alkyl-substituted aminoalkylsilanes or alkoxysilane-functional aspartic esters.
Modification of this kind, however, leads inevitably to a reduction in the average isocyanate functionality relative to that of the starting polyisocyanates. The effect of this reduction increases in proportion with the target silane content of the reaction product. In practice, however, in the above mentioned applications, such as coating materials or adhesives, for example, the specific desire, in order to obtain a high network density, is for polyisocyanate crosslinkers having a very high isocyanate functionality.
Moreover, as the degree of modification increases, there is also a drastic rise in the viscosity of the products, owing to the thiourethane groups and, more particularly, urea groups introduced into the molecule; for this reason, the existing polyisocyanates containing silane groups can generally only be employed in dissolved form using considerable quantities of organic solvents.
A further disadvantage of the existing preparation processes for such products is that the reaction of polyisocyanates with isocyanate-reactive organofunctional silanes leads to a random distribution of the silane functions over the oligomeric polyisocyanate mixture. As well as containing the desired silane-functionalized polyisocyanates, the reaction mixture always also includes unmodified starting polyisocyanate and—to an increasing extent with increasing degree of modification—completely isocyanate-free molecules, carrying exclusively silane groups as their reactive groups. When products of this kind are used as crosslinkers in polyurethane systems, the outcome is an inhomogeneous distribution of the silane units in the polymer backbone, and hence not the best-possible level of properties.